Optimizing the properties of granular walnut-shell based KOH activated carbons for carbon dioxide adsorption

•Granular ACs were successfully prepared using KOH activation of walnut shells.•Elevating HTT suppresses porosity while improves mechanical strength of materials.•Resultant ACs are promising adsorbents for CO2 adsorption/separation.•Micropores in size below 0.8 nm are the most effective in CO2 adsorption (1 bar,273 K).

A series of microporous granular activated carbons (GACs) has been prepared by KOH activation of walnut shell-based carbons. The characterization of GACs included the determination of microstrength and porous texture parameters (N2 adsorption at 77 K and CO2 adsorption at 273 K). The effect of walnut shell heat-treatment temperature (HTT) on GACs properties and CO2 uptake under pressure varying from sub-atmospheric up to 30 bar has been discussed. The study reveals that walnut shell HTT has to be at least 700 °C to provide a reasonable mechanical strength of GACs on activation at 800 °C and KOH:carbon ratio of 2:1. Elevating HTT from 700 to 900 °C suppresses porosity development (surface area SBET 2000 − 1300 m2 g−1, pore volume VT 0.82 − 0.56 cm3 g−1, micropore width L0 0.89-0.68 nm) while improves mechanical strength of GACs. Walnut shell-based GACs are promising adsorbents for CO2 adsorption/separation operating under different pressure. Suitable adapting HTT allows to prepare adsorbents with sufficient mechanical strength and high CO2 uptake (at 273 K up to 7.2 mmol g−1 under 1 bar and 18.2 mmol g−1 at 30 bar) with relatively low retentivity (15%) when degassed to 0.06 bar.